Further studies on argininosuccinase in various extrahepatic tissues of the same species are being continued. Immunological as well as enzymological criteria will be used for characterization in this comparative study. The work is undertaken with the aim of providing sensitive and useful methods of evaluation for questions of genetic interest involving human mutations affecting this enzyme. Investigations on the subunit composition of liver argininosuccinate synthetase and on the allosteric behavior and regulation of this enzyme at the molecular level are being continued. Studies on the regulation of liver argininosuccinase are being continued and investigations on the active site of this enzyme are planned.